1. Field of the Invention
The present invention relates to an image forming method using an image forming apparatus for an electrophotographic process, such as copiers, facsimiles and printers.
2. Description of the Related Art
Conventionally, in an image forming method for full-color electrophotography in a tandem system, colors are superimposed on a transfer medium. In the method, the second color toner image is superimposed on the unfixed first color toner image. The unfixed toner extremely unstably adheres to the transfer medium. The second toner image may be out of the predetermined position by a slight external force. Additionally, the second color toner is more unstably adheres to the transfer medium, because it is superimposed on the first color toner. When the toner adhesion to the transfer medium is unstable, the toner reversely transfers to a latent electrostatic image bearing member. As a result, a desired image density or color reproducibility cannot be obtained. Particularly, unstable color reproducibility (reverse transfer toner generation) caused by the unstable condition of the second color toner is a serious problem in a full-color process.
The mechanism of occurrence of the reverse transfer toner has not been elucidated, but generally, the reverse transfer toner is mainly considered to be caused by generation of the toner charged to have the opposite polarity to the normal toner charge polarity (hereinafter, referred to as the reverse polarity toner), the unstable average charge amount of toner, and generation of mechanical disturbing force between a transfer medium and an image bearing member. Specifically, it is considered to be caused by the following reasons: the electrical field causing the toner movement becomes more unstable in the second color toner than in the first color toner, and a toner layer on the transfer medium becomes thicker.
Generally, the charge amount of the toner which is transferred from an image bearing member to a transfer medium is preferably 10 μC/g to 30 μC/g in the absolute value. The charge amount of toner is an average value of the charge amounts of the measured toner, in fact, the charge amounts of toner have a distribution of a certain rang, and some toner particles have an opposite polarity to the normal toner polarity. As the amount of the reverse polarity toner increases, the amount of the reverse transfer toner tends to increase. Thus, a method of making the distribution of charge amount of toner sharp is generally used in order to prevent the increase of the amount of the reverse transfer toner.
Moreover, the larger the amount of toner transferred to the transfer medium, the more frequently reverse transfer occurs. For example, in a tandem system, the toner is more frequently reversely transferred in the image bearing member located in more downstream of the sheet conveyance direction.
For example, in order to reduce the reverse transfer toner, Japanese Patent Application Laid-Open (JP-A) No. 2005-31120 discloses a tandem image forming apparatus in which the reversely charged toner is recovered from the residual toner remaining on a photoconductor drum and retained and then the retained residual toner is returned to the photoconductor drum, wherein the toner used in a developing device which is located on the mostdownstream of a movement direction of an intermediate transfer belt, is adjusted to have the highest dielectric constant. Additionally, JP-A No. 2006-145805 discloses an image forming apparatus having a primary transfer roller configured to transfer a toner image on an intermediate belt, and a secondary transfer roller configured to transfer the toner image retained on the intermediate belt on variously sized recording media, wherein the differences in linear velocity and coefficient of dynamic friction between a photoconductor and the intermediate transfer belt are respectively 7 mm/s or more and 0.03 or more, and the primary transfer roller has a primary transfer pressure of less than 15N/m. Furthermore, JP-A 2005-338232 discloses an image forming apparatus containing a plurality of image forming units each configured to form toner images, and a transfer belt configured to transfer the toner images formed by the image forming units, in which the residual toner in one image forming unit arranged in the upstream of the medium conveying direction of the transfer belt which forms a medium conveying path, is moved onto the transfer belt, and then the moved residual toner is recovered to the other image forming unit arranged in the mostdownstream of the medium conveying direction of the transfer belt.